Purpose:
Visual attention is necessary to perceive and react to our world; when we do not attend something, we are often completely unaware of it. This lack of perception is exemplified in change blindness tasks in which we are unable to detect a difference between two scenes of objects separated by a blank screen, even though the change may be large. Past studies have suggested that behavioral effects of attention are due to modulation of neural activity in the visual cortex. Here we test this hypothesis by asking whether attentional modulation in visual area V4 can explain performance in a change detection task.

Methods:
Two animals were trained in a change detection task comprised of one, two, four, or eight orientated bars on a screen. The bars were shown to the animals twice for 500 ms, with a 100 ms gap between presentations. They had to spread their attention to all the bars and determine if one of had rotated between the presentations. In a control condition, one location was loaded with a high reward, which biased the animal’s attention towards that location.

Results:
The animals’ performance decreased as the number of stimuli increased. Given the results of past studies, we expected to see the neural activity of V4 correlate with the degree to which attention was spread; high activity correlating with less spread of attention and lower activity with more spread of attention. Using extracellular electrodes to measure spiking activity from single neurons, we found that the neurons in V4 did not vary as a function of the number of stimuli. When the animals were tested on the high reward task, attentional modulation in V4 was seen concurrent with improved performance at that location.

Conclusions:
Since the V4 activity did not vary as a function of the number of stimuli, but the behavior did, these results suggest that the attentional modulation seen in past studies is not responsible for the decreased performance seen with the greater set sizes. We believe the change in performance is due to a second physiological mechanism, which limits the information that can be passed forward to cognitive processing areas.